As wireless communication expands in terms of more and more users in the networks, the number of frequency carriers in a given area also has to be increased to maintain a certain degree of service. This means that Radio Frequency (RF) combining also has to be improved in order not to loose too much power. Combining carriers the classic way means a large ladder of 3-dB combiners with in the end a very low efficiency.
Another way to combine a large number of carriers is to combine at digital base-band or at some intermediate frequency at low signal power. The combined signal can then be jointly amplified using a broadband multi carrier power amplifier (MCPA).
A way of improving performance of MCPA's other than directly improving the linearity of for example transistors is to place a pre-distorter in front of the amplifier. There are a number of ways as how to accomplish this. One way is to pre-distort within MCPA itself. Usually this is done in an analog RF fashion. Such a solution is demonstrated for instance by U.S. Pat. No. 5,126,687. RF pre-distortion (PD) may also be done outside the full MCPA. Numerous documents are found describing different solution using pre-distortion. For instance JP, A, 01-314439 and JP, A, 59-017736 illustrates usage of pre-distorters.
Another way is to implement digital pre-distortion. Digital PD may be used whenever there is a digital combined signal at hand. For instance U.S. Pat. No. 6,072,364 illustrates such a digital solution The introduction of so-called software transceivers makes it possible to extract exactly this signal. On a system-level there would be a digital software transceiver, a broadband digital-to-analog converter (DAC), some RF hardware and the RF MCPA basically connected to the antenna port through an antenna feed cable. A digital pre-distorter would preferably be placed between the software transceiver and the DAC.
State-of-the-art of implementing pre-distorter is still in its cradle. Usually the technique is to enforce that the pre-distorter inherently reflects the nonlinear behavior of the MCPA itself. The details of the distortion curve are determined by a set of parameters, which may vary from model to model. The actual functionality of the pre-distorter is then to “bend” the non-linear curve the other way such as to counter-act the non-linear device.
The means of computing the parameters of the pre-distorter is usually to compare the input signal to the output signal, and then by some manner adjust the parameters in such a way as to minimize distortion at the output of the non-linear device. Usually this means a careful time or phase alignment of input-to-output signals, which makes it a difficult task to actually implement.
Moreover, it usually involves some sort of modeling of the pre-distorter in conjunction with the MCPA itself. In this way, the optimization procedure is somewhat two-fold in that it needs modeling of two items within the iteration procedure. It would be expected that the iteration process gets rather involved and also takes time to perform.
Therefore there is still a demand of an alternative method which may simplify the procedure of modeling the distortion compensation in conjunction with the MCPA itself avoiding the careful time or phase alignment of input-to-output signals, which normally makes a feed-back solution a difficult task to actually implement.